Measurement of Velocity Distributions and Recombination Kinetics in Tunnel-Ionized Helium Plasmas

We report the first measurements on the distribution of electron energies and associated recombination kinetics in tunnel-ionized plasmas of interest to recent x-ray laser proposals. Subpicosecond laser pulses, focused to an intensity of $1\times {10}^{16}$ W $/{\mathrm{cm}}^2$, create a fully ionized helium plasma. Measurement of the resulting emission spectrum allows a determination of the time-dependent electron velocity distribution. The initial distribution is found to be strongly non-Maxwellian and subsequent recombination kinetics proceed more slowly than predicted for a Maxwellian distribution of electrons.